Space Radiation

Ionizing radiation effects in spacecraft electronics can be separated into two areas: Single Event Effects (SEE) and Total Ionizing Dose (TID).

SEEs occur when a single ion strikes the IC, depositing sufficient energy in the device to cause an SEE. SEE may be divided into two main categories: soft errors and hard errors. A soft error occurs when a transient pulse or bitflip in the device causes an error detectable at the device output. Soft errors are entirely device specific, and are best categorized by their impact on the device.

Single Event Upset (SEU) is generally a transient pulse or bitflip. In logic or an A to D converter, a transient or spike on the device output would be a potential SEU; in a memory cell a bitflip would be an SEU. SEUs occurring in the device's control circuitry may also cause other effects. In general, resetting the device or rewriting the data corrects SEUs.

Hard errors can be physically destructive to the device, and cause permanent functional effects. Single Hard Error (SHE) causes a permanent change to the operation of the device. A common example would be a stuck bit in a memory device. Like SEUs, this is also device dependent. Single Event Latch up (SEL) is a potentially destructive condition. During a traditional or destructive SEL, the device current exceeds the maximum specified for the device. Unless power is removed, the device will eventually be destroyed.

The SEE sensitivity of a device is discussed in terms of Linear Energy Transfer (LET) and Cross Section (s). LET is a measure of the energy deposited per unit length as an ionizing particle travels through a material. The units for cross section are cm2 per device or per bit. Sensitive volume refers to the device volume affected by SEE-inducing radiation. (Calculation of LET and Range of All Ions in Any Material can be found at http://cyclotron.tamu.edu/ref/let.htm ) Copy and paste in your browser address line of click below.

TID comes from long-term degradation of electronics due to the cumulative energy deposited in a material. Effects include parametric failures, or variations in device parameters such as leakage current, threshold voltage, etc., and functional failures. Significant sources of TID exposure in the space environment include trapped electrons, trapped protons, and solar flare protons.


		LET Calculation				Radiation Sources